1. Field of the Invention
The invention relates generally to rotary drag bits, and more specifically to improvements in roof drill bit systems for drilling and boring as in roof bolting operations for tunnel construction and mining.
2. Description of the Prior Art
In the fields of industrial, mining and construction tools, polycrystalline diamond (PCD) is becoming more widely used in making cutting tool inserts, sometimes called polycrystalline diamond compacts (PDC). PCD materials are formed of fine diamond powder sintered by intercrystalline bonding under high temperature/high pressure diamond synthesis technology into a predetermined layer or shape; and such PCD layers are usually permanently bonded to a substrate of "precemented" tungsten carbide to form such PDC insert or compact. The term "high density ceramic" (HDC) is sometimes used to refer to a mining tool having an insert with a PCD layer. The term "chemical vapor deposition" (CVD) is a form of pure PCD that may be used for denser inserts and other super abrasive hard surfacing and layering materials, such as layered "nitride" compositions of titanium (TiN) and carbon (C.sub.2 N.sub.2), are gaining acceptance in the mining field. All such "hard surface" materials--PCD, CVD and nitride compositions as well as titanium carbide and other more conventional bit materials are applicable to the present invention and considered alternatives unless specifically distinguished from each other herein. Some of the basic underlying technology pertaining to PCD materials is disclosed in U.S. Pat. Nos. 4,525,178; 4,570,726; 4,604,106; and 4,694,918.
The principal types of drill bits used in rotary drilling operations are roller bits and drag bits. In roller bits, rolled cones are secured in sequences on the bit to form cutting teeth to crush and break up rock and earth material by compressive force as the bit is rotated at the bottom of the bore hole. In drag bits, PCD cutting elements on the bit act to cut or shear the earth material. The action of some flushing fluid medium, such as fluid drilling mud, water or a compressed air/vacuum system, is important in all types of drilling operations to cool the cutting elements and to flush or transport cuttings away from the cutting elements and remove them from the hole. It is important to remove cuttings to prevent accumulations that may plug water passages and "ball up" or otherwise interfere with the crushing or cutting action of the bit, and the cooling action is particularly important in the use of PCD/CVD cutters to prevent carbon transformation of the diamond material.
Roof drill bits are a form of rotary drag bit and are used in roof bolting operations, which are overhead so the drilling operation is upward. In most cases the earth structure is formed of extremely hard rock or mineral (coal) deposits, although stratas of shale, loose (fractured) rock and mud layers are frequently encountered in boring or drilling operations for roof bolting construction. The use of large quantities of water (drilling mud) is typical in roof drilling to cool the cutting elements and flush the cuttings away, but overhead irrigation results in uncontrolled water loss and floor flooding that make working conditions unsafe and unpleasant. It should also be recognized that the presence of methane gas in coal mines and the like constitutes a safety hazard, and respirable dust is a further safety consideration in the mining industry. In a typical roof bolting operation, a series of 4 foot to 6 foot holes having a diameter of 3/4 inch to 2 inches (or more) are drilled in the tunnel roof to receive bolts for anchoring roof support structures. In the past tungsten carbide bits frequently could only drill a single 4 foot hole before the bit became dull or broken. It should be noted also that where long flexible cable roof bolts are used as for some soft earth formations, 12 foot to 24 foot holes may be required and it may take up to 30 minutes to drill a single hole using prior art tungsten carbide drill bits.
My prior U.S. Pat. Nos. 5,180,022; 5,303,787 and 5,383,526 disclose substantial improvements in HCD roof drill bits using PCD cutting elements constructed in a non-coring arrangement, and also teach novel drilling methods that greatly accelerate the speed of drilling action and substantially reduce bit breakage and change-over downtime. My prior HCD non-coring drill bits are capable of drilling over 100-300 holes of 4 foot depth with a single bit and in shorter times with less thrust than the standard carbide bits in hard rock formations of 22,000-28,000 psi. However, although my prior HCD non-coring bits easily drill through earth structures that include shale, mud seams and other broken and soft formations, it has been discovered that these drill bits tend to plug in drilling through mud seams and other soft or broken earth formations and PCD cutting inserts may even shatter in working through stratas of extremely hard, fractured and muddy earth conditions.
Comparative tests conducted in three states have determined that the amount of water required to wet drill with PCD rotary bits may be reduced from a conventional (tungsten carbide bit) range of 9-18 gallons per minutes down to about 1-3 quarts per minutes when atomized into an air mist that effectively scours and cools the PCD inserts. Wet drilling in non-recoverable drilling operations currently being used achieves a penetration of 6-9 ft./min. requiring 6-9 gal./min. at 90 psi or 9-14 gal./min. at 150 psi or 18 gal./min. at 300 psi. Experimental testing in West Virginia was in fairly solid, 65% quartz sandstone with some 4 inch mud seams using HDC rotary bits and in air-water jet mist; the result achieved a penetration rate of 12 ft./min. with no plugging as compared with usual 6-9 ft./min. penetration using only water as the flushing agent. In Utah, experimental testing was conducted in a very muddy sandstone top with 20% silica content using 11/32 inch HDC of drill bits and 100-120 psi air-water mist. Prior conventional drilling of each 6 foot hole in this mine with water only was timed at 4-6 minutes, as compared to 45-70 seconds by using the air-water mist of the present invention. The U.S. Bureau of Mines ordered an independent test relative to respirable dust generated in drilling quartz sandstone; it was determined that a substantial reduction in respirable dust results from using the air-water jet mist over the use of air per se.
In comparing the air-water jet mist to prior art "water only" flushing, it should be emphasized that the present invention utilizes only about 3 qts./min./drill column as compared to 6-9 gals./min. resulting in water savings into the millions of gallons range per mine each year.